For the first time, I had an inspector note that he wanted the sanitary system to be capable of handling all of the continuous backflow preventer water flow if there was a catastrophic release of water. In order to accomodate this, we had to replace the backflow preventer with two 3/4" backflow preventers and make the assumption that only one of them would discharge at a time. The other alternative would have been to replace the 4" sanitary line with a 10" line, which is ridiculous. Any one else out there interpret the UPC (or IPC) this way? Any one else have other ideas on how to meet this requirement? I know New York and probably other jurisdictions have been requiring this, so I hope someone out there can help.

A bit confusing. IF you could replace a single BFP with two 3/4" ones, then the original one could not have been much more than 1", and a 1", or even 1 1/4", BFP would NEVER overwhelm a 4" sewer line, and would absolutely NOT require a 10" sewer. The assumption that "only one of them would backflow at a time" would ONLY be valid if they were on two different systems, (which could not be the case if the two valves are replacing a single one)), otherwise the same conditions which would cause one to backflow would also operate the second one at the same time. The other question would be, "Where is all of this water coming from", unless you have two separate systems tied together. Otherwise the only "backflow" should be whatever water is contained in the downstream, usually closed, system. Did you leave out some critical information?

As far as the "only one of them would backflow at a time", they are not on seperate systems. The Inspector has approved similar projects where two smaller backflow preventers were used, so it is his assumption that only one will discharge at a time.

I think in the Watts tables, this is a catastrophic discharge where the flow is coming from the water main side.

Maybe this can clarify my question. In the past, we have put in sanitary drainage from backflow preventers to handle the water discharge if the backflow preventer stopped backflow into the main. This is not a huge amount of water and also the system would eventually drain - so this is no problem. This inspector wanted the sanitary system to handle a catastrophic failure where water is coming from the main side - which is a much higher flow and will not stop.

The UPC says that "Drain lines serving backflow devices or assemblies shall be sized in accordance with the discharge rates of the manufacturer's flow charts of such devies or assemblies". This has always meant the discharge rate when preventing backflow, but now it appears it is for a catastorphic failure. This is going to add to owner cost - either with two BFP's and the associated annual cost or in much larger sanitary piping. Has anyone else seen this requirement? Any ideas for future projects? Thanks!!

How did you EVER equate a 2" BFP with TWO 3/4" ones? A 2" line would require at least FOUR 1" valves, and probably five, to equal the capacity. Whatever caused one to operate would also make the other one do the same at the same time. You are assuming the "air gap" on the BFP, which I hope you are using, would accept that "full flow" without spilling onto the floor, which is NOT likely to happen since it is also only 2" or less and operating on gravity flow the same as the main sewer. Your engineering appears to be flawed.